Soil is a typical porous media and its impedance characteristic directly determines the performance of grounding system. Soil phase frequency characteristic measurements were carried out on various soil types and wate...Soil is a typical porous media and its impedance characteristic directly determines the performance of grounding system. Soil phase frequency characteristic measurements were carried out on various soil types and water content. This paper finds that the impedance angle of soil specimen presents a capacitive performance when power frequency (f) is low. As the frequency increases, soil impedance angle goes up rapidly. Furthermore the frequency characteristic while f > 1000 Hz is distinct in terms of different water content. In particular, at low moisture content, soil impedance angle would be higher than 0?, that is, the inductive component is obvious. The study result indicates that porous media possesses the unique conductivity property dif-ferent from conductor and solution. Its mechanism needs further study.展开更多
随着双馈风电和高压直流输电(high-voltage direct-current,HVDC)在电力系统中的比例越来越大,电网换相换流器型HVDC(line-commutated converter based HVDC,LCC-HVDC)与双馈风电场、弱电网互联时会存在交互稳定问题,且振荡频率会呈现...随着双馈风电和高压直流输电(high-voltage direct-current,HVDC)在电力系统中的比例越来越大,电网换相换流器型HVDC(line-commutated converter based HVDC,LCC-HVDC)与双馈风电场、弱电网互联时会存在交互稳定问题,且振荡频率会呈现频率耦合特征。针对LCC-HVDC整流站存在的频率耦合现象,采用3-D傅里叶变换建立阻抗模型,分析LCC产生频率耦合现象的机理,进行各个影响因素对LCC频率耦合程度的敏感性分析。最后,研究频率耦合特性对LCC整流站、双馈风场、弱电网互联系统稳定性分析的影响,并通过时域仿真验证所得结论。展开更多
In this work, a numerical study for designing a new kind of MHD （Magneto-Hydrn-Dynamic） pumps is presented. This technique makes a compromise between electrolysis prevention and high flow rate performance. This tech...In this work, a numerical study for designing a new kind of MHD （Magneto-Hydrn-Dynamic） pumps is presented. This technique makes a compromise between electrolysis prevention and high flow rate performance. This technique should eliminate electrolytic bubble generation, electrodes wear and fluid propriety modification. All these side phenomena are prevented by considering isolated electrodes. The numerical presented results in this paper demonstrate that continuous MHD pumping is possible with isolated electrodes. The MHD excitation combines a high frequency altering current with a low frequency altering magnetic field. In order to validate our results, two independent theoretical methods for computing flow rate are followed. The two presented independent approaches show that high flow rate is possible even with isolated electrodes. To overcome the problem of dimensioning this kind of pumps, a generic numerical analysis is proposed. Hence, the pump performances as functions of the external parameter are studied and tools to calculate for a given fluid and the optimal high frequency regime are provided.展开更多
高压直流(HVDC)换流器阻抗频率特性的确定对避免系统发生谐振具有重要意义。文中提出了基于开关函数理论的HVDC换流器直流等值阻抗和交流等值阻抗计算方法。该方法计及了换流器换相过程的影响,计算换流器的直流等值阻抗时考虑了交流系...高压直流(HVDC)换流器阻抗频率特性的确定对避免系统发生谐振具有重要意义。文中提出了基于开关函数理论的HVDC换流器直流等值阻抗和交流等值阻抗计算方法。该方法计及了换流器换相过程的影响,计算换流器的直流等值阻抗时考虑了交流系统的影响,计算换流器交流等值阻抗时考虑了直流系统乃至对侧交流系统的影响。CIGRE first benchmark模型被用于验证该方法。PSCAD/EMTDC仿真和该方法计算得到的阻抗频率特性相吻合。仿真结果证明,直流系统对交流系统的谐振频率有着不可忽略的影响,这种谐振有可能导致直流输电系统的不稳定运行, 而该方法可以准确、方便地计算出谐振频率。展开更多
文摘Soil is a typical porous media and its impedance characteristic directly determines the performance of grounding system. Soil phase frequency characteristic measurements were carried out on various soil types and water content. This paper finds that the impedance angle of soil specimen presents a capacitive performance when power frequency (f) is low. As the frequency increases, soil impedance angle goes up rapidly. Furthermore the frequency characteristic while f > 1000 Hz is distinct in terms of different water content. In particular, at low moisture content, soil impedance angle would be higher than 0?, that is, the inductive component is obvious. The study result indicates that porous media possesses the unique conductivity property dif-ferent from conductor and solution. Its mechanism needs further study.
文摘随着双馈风电和高压直流输电(high-voltage direct-current,HVDC)在电力系统中的比例越来越大,电网换相换流器型HVDC(line-commutated converter based HVDC,LCC-HVDC)与双馈风电场、弱电网互联时会存在交互稳定问题,且振荡频率会呈现频率耦合特征。针对LCC-HVDC整流站存在的频率耦合现象,采用3-D傅里叶变换建立阻抗模型,分析LCC产生频率耦合现象的机理,进行各个影响因素对LCC频率耦合程度的敏感性分析。最后,研究频率耦合特性对LCC整流站、双馈风场、弱电网互联系统稳定性分析的影响,并通过时域仿真验证所得结论。
文摘In this work, a numerical study for designing a new kind of MHD （Magneto-Hydrn-Dynamic） pumps is presented. This technique makes a compromise between electrolysis prevention and high flow rate performance. This technique should eliminate electrolytic bubble generation, electrodes wear and fluid propriety modification. All these side phenomena are prevented by considering isolated electrodes. The numerical presented results in this paper demonstrate that continuous MHD pumping is possible with isolated electrodes. The MHD excitation combines a high frequency altering current with a low frequency altering magnetic field. In order to validate our results, two independent theoretical methods for computing flow rate are followed. The two presented independent approaches show that high flow rate is possible even with isolated electrodes. To overcome the problem of dimensioning this kind of pumps, a generic numerical analysis is proposed. Hence, the pump performances as functions of the external parameter are studied and tools to calculate for a given fluid and the optimal high frequency regime are provided.
文摘高压直流(HVDC)换流器阻抗频率特性的确定对避免系统发生谐振具有重要意义。文中提出了基于开关函数理论的HVDC换流器直流等值阻抗和交流等值阻抗计算方法。该方法计及了换流器换相过程的影响,计算换流器的直流等值阻抗时考虑了交流系统的影响,计算换流器交流等值阻抗时考虑了直流系统乃至对侧交流系统的影响。CIGRE first benchmark模型被用于验证该方法。PSCAD/EMTDC仿真和该方法计算得到的阻抗频率特性相吻合。仿真结果证明,直流系统对交流系统的谐振频率有着不可忽略的影响,这种谐振有可能导致直流输电系统的不稳定运行, 而该方法可以准确、方便地计算出谐振频率。
